The present invention relates to a method and an arrangement for monitoring connections of a switch intended for activating a safety function of a device.
An inverter is an electric device which enables a voltage with an alterable frequency to be formed. Typically, inverters are used in connection with motors for controlling the motors at a variable frequency. An inverter may also be a part of a frequency converter which is used e.g. for controlling a motor. In connection with inverters or frequency converters generally comprising inverters, various safety functions are generally applied with the purpose of monitoring and ensuring the safety of e.g. motor drives in different situations and conditions of use. One such safety function is Safe Torque Off (STO). The Safe Torque Off is a functionality defined in IEC61508 standard, whose instructions enable safe devices to be designed in a reliable manner. A desired level for frequency converters is SIL (Safety Integrity Level) 3.
For instance in a frequency converter, the STO function is to ensure that no torque is formed for the motor controlled by the frequency converter if the safety switch has been turned. In the case of the frequency converter application, a user is typically left with the task of installing the pair cables received by the connectors of the STO connection as well as the safety switch having two electrically separate switches. FIG. 1 shows an example of an ordinary coupling of a safety switch 20 associated with the STO function to an inverter 10 of a frequency converter. The figure shows only one phase out of the three phases of the output stage of the inverter, which supplies the motor via phases U2, V2 and W2. The output stage of the inverter is controlled by a control circuit 11 by means of gate drivers 12a and 12b. The STO connection of an inverter (or a frequency converter) is typically provided with four terminals 1, 2, 3, 4 of which two 1, 2 usually have an active voltage supply, e.g. 24 VDC. From these terminals 1, 2 the voltage supply is led to the terminals 3, 4 via the two-pole safety switch 20 and connections 30 and 40. From the terminals 3, 4 the voltage is further supplied e.g. to a control circuit 11 and to be the operating voltage of the gate drivers 12a, 12b. The voltage supply is thus supplied via the safety switch S2 as well as the connections 30 and 40 through two separate connection branches. The task of the safety switch 20 is to cut off this voltage supply, as a consequence of which the STO logic/function has to prevent a moment from being formed to the motor being supplied by the inverter 10. In the exemplary configuration of FIG. 1, opening of the safety switch S2 cuts off both connection branches and thus the voltage supply to the control circuit of the inverter 10 as well to the gate drivers 12a, 12b. 
A problem with the above-described coupling of the safety switch is that the pair cable used by the user for the connection of the safety switch may be defective or it may become defective in use. It is therefore advisable to instruct the user to use a sheathed cable for the connections of the safety switch so that e.g. a defective cable that has become squeezed would cause an STO function. However, it may be difficult to find sheathed cables and, in addition, it is difficult to ensure that the user eventually uses an appropriate sheathed cable and grounds the sheath. In the worst case, an ungrounded sheath may short-circuit the conductors of the connection cable which, in turn, may lead to a situation wherein e.g. when one switch is broken the STO function does not work at all. Further, it cannot be ensured that the user couples the switch correctly and uses a two-pole switch.